The present invention relates to combining digital key signals, to form a composite digital key signal for combining a first digital video signal with a second digital video signal.
Systems are known in which a first video image may be combined with a background video image in real time, to produce a real time video output signal. A known application of this technique involves removing a particular object from a first video image and placing said object against a different background. In order to remove the object, a particular characteristic of the signal, representing the object, may be selected, such as its color or its brightness. A first known technique generates a key in response to the presence of a particular color and this technique is commonly referred to as chroma keying. Alternatively, the technique of using brightness levels to generate a key signal is referred to as luminance keying. Thus, in chroma keying, a key signal may be derived from all areas of the picture in which a particular color is present (usually a saturated blue) and the output signal is produced by adding the two input signals together, after the background image has been multiplied by the value of the key k and the foreground image has been multiplied by the key value subtracted from unity, 1-k.
Keying is also employed when a video signal is manipulated by a video effects machine. Video effects machines are arranged to manipulate complete frames to produce an image which is smaller than the full video picture. The manipulated image is then usually placed against a background image and, therefore, the video effects machine must generate a key signal defining the shape of the manipulated image, which is then used to combine the manipulated image with a background image, using two multiplying stages and an adding stage, as described above.
In recent years, in both television broadcasting and video post-production, there has been a move away from conventional analog processing and towards digital processing. Thus, original video recordings may be made using a digital format, or photographic films may be converted to digital video, whereafter all of the mixing and post-production manipulations may be performed in the digital domain, thereby allowing many more manipulations to be made without degrading the integrity of the video signal.
A digital video signal defines an array of discrete picture point data and, unless great care is taken, images may be created which cannot be properly represented by the picture points, resulting in aliasing which expresses itself in the form of visible jaggies. Thus, for example, key signals may be produced which have very sharp transitions and, unless said transitions occur at boundaries between adjacent picture points, they cannot be properly represented by the digital signal and undesirable artefacts will be introduced. It is known to produce key signals with so-called "soft" edges, in which a transition between two extreme levels occurs over a plurality of picture points, such that a boundary exists over which, rather than a sharp transition occurring between the two images, the images actually merge together and, although an actual borderline between the images does not exist, such a border is still perceived by the eye, to the extent that the newly-combined foreground and background components are accepted as being part of a common image.
With techniques available for combining two video images, demand has grown for systems which are capable of combining more than two images. However, a particular problem which is encountered when attempting to provide such a facility is that a composite key must be produced for keying previously combined images against additional images or against a background. In accordance with a known technique, signals may be combined by multiplication. However, if two linear keys, having values ranging between zero and unity, are multiplied together, the result is non-linear and if such a non-linear key is used to combine partial images against the background, a visible border may be produced between the combined images and the background. Furthermore, as more and more key signals are combined together in this way, undesirable artefacts of this type become more and more apparent, easily resulting in a totally unacceptable end-product.
An alternative approach to combining key signals is to add the key signals together. However, if two key signals are to be combined which lie within the range of zero to unity and the output is also to lie within this range, sums greater than unity, when produced, will result in the new key saturating. Thus, a result of such a process is to effectively increase the gradient of the key signal which, although producing a linear key, produces harder edges which in turn may introduce artefacts due to aliasing, as previously described. Again, as the number of key signals added together increases, the problem becomes worse and worse and soon becomes clearly visible in the final picture.
It is an object of the present invention to provide an improved apparatus for combining digital key signals to form composite digital key signals. In particular, it is an object of the present invention to provide an apparatus for the multiple combination of digital key signals, without introducing undesirable artefacts.